Light olefins such as ethylene, propylene, butenes, butadiene are produced from the pyrolysis of hydrocarbons at high temperatures (700° C. and above) and low pressures (at or slightly above atmospheric). Conventional pyrolysis processes, such as steam cracking, can be utilized to do this. Other co-products include steam cracked naphtha (SCN), steam cracked gas oil (SCGO) and steam cracked tar (SCT).
Steam cracking of hydrocarbons has typically been effected by supplying the hydrocarbon feedstock in vaporized or substantially vaporized form, in admixture with substantial amounts of steam, to suitable coils made up of tubes in a pyrolysis furnace. It is conventional to pass the reaction mixture through a number of parallel coils which pass through a convection section of the pyrolysis furnace wherein hot combustion gases raise the temperature of the reaction mixture. The reaction mixture then passes through a number of specially designed radiant coils made up of tubes in a radiant section enclosure of the pyrolysis furnace wherein a multiplicity of burners supply the heat necessary to bring the reactants to the desired reaction temperature and effect the desired reactions. Undesirable byproduct molecules from the pyrolysis include coke and asphaltenes. The asphaltene molecules are undesirable because they can foul the surfaces in the process as they condense, and are generally low valued. A substantial amount of the coke deposits on surfaces in the pyrolysis reaction system and eventually must be removed by de-coking.
Pyrolysis furnaces used in steam cracking present some of the most severe operating conditions encountered in the chemical process industries. In addition to the high operating temperatures, the tubes experience coking, carburization, oxidation, creep and thermal cycling during operations. Over the years, furnace temperatures have tended to rise to improve feedstock conversion and desirable product yields, placing increasingly severe operating conditions on the pyrolysis tubes.
An important concern in hydrocarbon cracking processes, such as steam cracking, is the formation of coke. When hydrocarbon feedstocks are subjected to the heating conditions prevalent in a steam pyrolysis furnace, coke deposits tend to form on the inner walls of the tubes forming the cracking coils. Such coke deposits interfere with heat flow through the tube walls into the stream of reactants and raise the tube metal temperature.
A variety of heat-resistant alloy steels have been developed for use in pyrolysis furnaces. Although it is well-known that alloy steels containing a relatively high content of chromium and nickel are useful in constructing heat-resistant pyrolysis tubes having relatively long performance lives, premature tube failure continues to be a problem. One cause of such failure is carburization of the tubes brought about by the extremely high temperatures and carburizing atmospheres encountered. Carburization of such tubes, which results from the diffusion of carbon (e.g., from coke) into the alloy steel, causing the formation of additional carbides, brings about the embrittling of the steel. Once the steel has become embrittled, it is more susceptible to creep rupture failure, and/or brittle fracture due to thermal stress. Carburization often occurs at localized spots in the tubes, and of course when this has proceeded to the point of failure or potential failure, even at only one spot, the tubing must be replaced.
The radiant section coils are fabricated by joining two or more tubes typically by welding. Tube failure often occurs at or near the location of welds joining two tubes. This problem is worsened when relatively high temperature is needed to accomplish the pyrolysis, such as when the feed comprises one or more of ethane, propane, gas oil, crude oil, or other heavy oil.
U.S. Pat. No. 6,719,953 discloses an internally finned U-tube coil, a number of which are enclosed in a fired heater radiant section, and utilization of the same in a process for producing olefins form hydrocarbon feedstocks. This patent discloses at column 6 lines 62-67, an intermediate weld at the bottom of the U-tube coils where the weld is shielded from direct radiation by the adjacent coils. The fins are aligned at this connection.
U.S. Pat. No. 4,827,074 discloses a butt-weld for internally-finned steam cracker tubes. The tubes are utilized for steam cracking naphtha. The patent discloses that a conical counterbore of ≦75°, preferably in the range of 8° to 30°, lessens the accumulation of coke during steam cracking. The length of the cylindrical counterbore L/2 is fixed in a specified range, the range depending on the average distance along a diameter between the outside of the tube and the bottom of the tube's fins (grooves) and the average height of the fins. When L is smaller than this range, coke is observed to adhere in the counterbore region. When L is larger than this range, high turbulence leads to hot-spots.
There is still a need for an improved tube joint technology to reduce the frequency of radiant tube failure seen in pyrolysis furnaces, particularly for pyrolysis tubes containing a relatively high content chromium and nickel, such as those utilized at a relatively high pyrolysis temperature.